Maybe parent low poly objects to a spinning empty that is parented to the tornado. Several empties could be used with different rotation Ipo curves so although more than one object is parented to a single empty, not all objects are spinning at the same rate.
This might be an acceptable way to fake the vortex, depending on the results you want.
I think scaling the empty would move the objects towards the centre or further out and moving it along the vertical axis would make objects move up/down.
There is always the option of scripting a particle system, but the method above might work just using Ipo curves.
Edit: Just read a bit more thoroughly and this is quite similar to Blendiac’s suggestion. We seem to agree that ‘faking it’ would be better/easier than an actual particle system though.